Conventionally, there has been developed a two-dimensional image sensor (image reading device) of an active-matrix type in which a photo-detecting element (photo diode, photo transistor) and a switching element (thin-film transistor) are disposed on a two-dimensional plane, as disclosed in Japanese Utility Model Application No. 8055/1990 (Jitsukaihei 2-8055; published on Jan. 18, 1990).
Such a two-dimensional image sensor has an active-matrix array (active-matrix substrate), as shown in FIG. 9, including source lines s1, s2, . . . , sm, and gate lines g1, g2, . . . , gn intersecting with each other, and pixels that are arrayed in an X-Y matrix at the intersections of these lines. Each pixel has a photo-detecting TFT (photo-detecting thin-film transistor), a switching TFT (switching thin-film transistor), and a capacitor.
The two-dimensional image sensor reads a document image by projecting light onto a document while the resistance across the source and drain electrodes are high, i.e., when the photosensor TFT is OFF with the voltage of the gate electrode set to Vg1. When the reflected light from the surface of the document is incident on the photosensor TFT, the resistance value of the photosensor TFT decreases, causing the source-drain current to vary from a dark current (Idark) to a photo current (Iphoto), as shown in FIG. 10.
By taking advantage of this principle; the magnitude of source-drain current is varied according to the brightness of the irradiated object (document), i.e., the reflectance of the light. The difference between Idark and Iphoto determines the amount of charge stored in or discharged from the capacitor of each pixel.
The switching TFTs are used to sequentially read the charge distribution (in-plane distribution) of the capacitors, thereby obtaining two-dimensional information of the object.
However, conventional image reading devices as exemplified by the foregoing two-dimensional image sensor have the following problem.
In conventional image reading devices, when the driving voltage of the Cs electrode maintains the same polarity throughout the reading cycles (frames), stress caused by a DC-bias is applied to the TFT element or capacitor when the device is used for an extended time period, with the result that the electrical characteristics of the TFT element or capacitor are changed.
More specifically, the potential of the capacitor fluctuates or the polarity symmetry is disturbed, though to a limited extent, by the influence of the charge captured in an insulating film as a result of the DC-bias stress.
Thus, conventional image reading devices pose the problem of reliability when used over extended periods of time.